1. Field of the Invention
The invention relates to apparatus and methods associated with color measurement and analysis technology and, more particularly, to apparatus and methods for automated color density measurements of color "control bars" and the like obtained during a scanning procedure.
2. Description of Related Art
It is well known that the term "color" as applied to electromagnetic radiation represents in part the relative energy distribution of radiation within the visible spectrum. That is, light providing a stimulus to the human eye, and having a particular energy distribution, may be perceived as a substantially different color than light of another energy distribution. Concepts relating to the characteristics of color and light waves are the subject of numerous well-known texts, such as Principles of Color Technology, Meyer, Jr. and Saltzman (Wiley 1966), and The Measurement of Appearance, Hunter and Harold (Wiley 2nd Ed. 1987).
In recent years, the capability of maintaining the "quality" of color has been of significant importance in various industries, such as, for example, the fields of graphic arts, photography and color film processing. For purposes of performing sample testing and other activities in furtherance of maintaining color quality, it is necessary to first determine an appropriate means for "measuring" and "describing" color. A substantial amount of research has been performed during the past fifty years with respect to appropriate methods and standards for color measurement and description.
For purposes of describing color, and from a purely "physical" point of view, the production of color requires three things: a source of light; an object to be illuminated; and, a means for perceiving the color of the object. The means for perceiving the color can be the human eye and brain or, alternatively, electrical and electromechanical apparatus such as a photosensitive detector and associated auxiliary devices utilized for detecting light. In general, it is desirable to provide a means for measuring color so as to assess the manner in which an image will appear to a human observer, or the manner in which an image will perform in a photographic or other type of reproduction printing operation.
Although human perception and interpretation of color can be useful, reliance on such perception and interpretation can be highly subjective. That is, human nature may cause one person's perception of the color of a particular object to be substantially different from the perception of another. In addition, eye fatigue, age and other physiological factors can influence color perception. Further, visual human perception is often insufficient for color description. For example, certain object samples may be visually perceived under one light source as substantially "matching", and yet may actually have very different spectral characteristics and may be perceived as "nonmatching" under another light source. In view of the foregoing, it is desirable to employ color measurement and description techniques which are objective in nature, and capable of differentiating among object samples having different color characteristics.
Various devices have been developed and are widely utilized to measure and quantitatively describe color characteristics of object samples. Many of these devices provide measurements related to the spectral characteristics of the samples. Described simplistically, when light is directed onto an object sample to be measured for color, the object may absorb a portion of the light energy, while correspondingly passing through or reflecting (if the object is opaque) other portions of the light. The color characteristics of the object sample will depend in part on the spectral characteristics of the object. That is, the effect of an object on light can be described by its spectral transmittance or reflectance curves (for transparent or reflective materials, respectively). These spectral characteristic curves indicate the fraction of the source light at each wavelength transmitted by or reflected from the materials. Such curves are a means for describing the effect of an object on light in a manner similar to the use of a spectral energy distribution curve for describing the characteristics of a source of light. Instruments utilized for generating such spectral characteristics curves are typically referred to as spectrophotometers.
In accordance with conventional optical physics, it is known that the proportion of light incident to an object sample and absorbed by such a sample is independent of the light intensity. Accordingly, a quantitative indication of the spectral characteristics of an object sample can be defined as the "transmittance" or "reflectance" of the sample. That is, the transmittance of a substantially transparent object can be defined as the ratio of power transmitted over light power incident to the sample. Correspondingly, for a reflective object sample, the reflectance can be defined as the ratio of power reflected from the object over the incident light power.
For collimated light, these ratios can be expressed in terms of intensities, rather than power. Furthermore, because of the nature of transmittance/reflectance and the optical characteristics of the human eye, it is advantageous to express these ratios in logarithmic form. Accordingly, one parameter widely used in the field of color technology for obtaining a quantitative measurement or "figure of merit" is typically characterized as optical "density." The optical density of an object sample is typically defined as follows: EQU Optical Density=D=-log.sub.10 T or -log.sub.10 R (Equation 1)
where T represents transmittance of a transparent object and R represents reflectance of a reflective object. In accordance with the foregoing, if an object sample absorbed 90% of the light incident upon the sample, the reflectance would ideally be 10%. The density of such a sample would then be characterized as unity. Correspondingly, if 99.9% of the light were absorbed, the reflectance would be 0.1% and the density would be 3. Similarly, the density of an "ideal" object reflecting 100% of the light incident upon the object would be 0.
To provide a relative measurement of color, it is possible to utilize the principles of optical density, without requiring measurement or knowledge of the absolute values of total incident light intensity or reflectance. That is, for example, it is possible to obtain relative color measurements among a series of object samples by utilizing a particular geometric configuration of light, object sample and reflectance or transmittance detector for each measurement, and standardizing the measurements in some desired manner.
In brief summary, optical density is a measurement of the modulation of light or other radiant flux by an object sample, such as a particular "patch" of a color "control bar" conventionally employed in the printing and graphic arts industries. Density measurements provide a means to assess the manner in which an image will appear to a human observer, or the way an image will perform in a film processing operation. Density measurements can be utilized to produce sensitometric curves to evaluate various printing and reproduction characteristics, as well as utilization to control various photographic operations, such as film processing.
For purposes of measuring optical densities, it is well known to employ a device typically characterized as a "densitometer." These densitometers are often categorized as either "reflection" densitometers, employed for optical density measurements of reflective objects, or are otherwise characterized as "transmission" densitometers. Transmission densitometers are employed for determining spectral characteristics of various light transmitting materials.
Densitometers are utilized in various industries for performing a variety of functions. For example, densitometers can be conveniently employed in printing and graphic arts applications. Processes associated with these applications will be described in greater detail in subsequent paragraphs herein.
To assist in describing the principles of densitometer apparatus, in which certain concepts of the present invention may be employed, FIG. 1 illustrates a simplified schematic representation of a known reflection densitometer configuration 1. Densitometer apparatus of the type shown in FIG. 1 are characterized as reflection densitometers, and utilized to provide color density measurements of reflection materials as previously described.
Referring specifically to FIG. 1, and to numerical references therein, the densitometer apparatus 1 includes a light source unit 2 having a source light 4. With respect to optical density measurements in printing, color film processing, and other industrial fields, various standards have been developed for densitometer light source illuminants. For example, densitometer light source standards have previously been described in terms of a tungsten lamp providing an influx from a lamp operating at a Planckian distribution of 3000.degree. K. Other suggested standards have been developed by the American National Standards Institute ("ANSI") and the International Organization for Standardization ("ISO"). These source light densitometer standards are typically defined in terms of the spectral energy distribution of the illuminant. The source light 4 preferably conforms to an appropriate standard and can, for example, comprise a filament bulb meeting a standard conventionally known in the industry as 2856K ANSI. Power for the source light 4 and other elements of the densitometer apparatus 1 can be provided by means of conventional rechargeable batteries or, alternatively, interconnection to AC utility power for many known densitometers.
The source light 4 projects light through a collimating lens 6 which serves to focus the electromagnetic radiation from the source light 4 into a narrow collimated beam of light rays. Various types of conventional and well-known collimating lenses can be employed. The light rays transmitted through the collimating lens 6 project through an aperture 8. The dimensions of the aperture 8 will determine the size of the irradiated area of the object sample under test.
Various standards have been defined for preferable sizes of the irradiated area. Ideally, the aperture 8 is of a size such that the irradiance is uniform over the entire irradiated area. However, in any physically realizable densitometer arrangement, such uniform irradiance cannot be achieved. Current standards suggest that the size of the irradiated area should be such that irradiance measured at any point within the area is at least 90% of the maximum value. In addition, however, aperture size is typically limited to the size of the color bar or color patch area to be measured, and is also sized so as to reduce stray light.
The light rays emerging from the aperture 8 (illustrated as rays 10 in FIG. 1) are projected onto the irradiated area surface of an object sample 12 under test. The sample 12 may be any of numerous types of colored reflective materials. For example, in the printing industry, the sample 12 may be an ink-on-paper sample comprising a portion of a color bar at the edge of a color printing sheet. Alternatively, the sample 12 may be a control strip employed in the color film processing industry.
As the light rays 10 are projected onto the object sample 12, electromagnetic radiation shown as light rays 14 will be reflected from the sample 12. Standard detection configurations have been developed, whereby reflected light is detected at a specific angle relative to the illumination light rays 10 projected normal to the plane of the object sample 12. More specifically, standards have been developed for detection of reflected light rays at an angle of 45.degree. to the normal direction of the light rays 10. This angle of 45.degree. has become a standard for reflectance measurements, and is considered desirable in that this configuration will tend to maximize the density range of the measurements. In addition, however, the 45.degree. differential also represents somewhat of a relatively normal viewing configuration of a human observer (i.e. illumination at a 45.degree. angle from the viewer's line of sight).
For purposes of providing light detection, a spectral filter apparatus 16 is provided. The filter apparatus 16 can include a series of filters 18, 20 and 22. The filters 18, 20 and 22 are employed for purposes of discriminating the red, green and blue spectral responses, respectively. To more fully explain, red light is absorbed by a cyan ink, thereby providing a cyan color appearance to the observer. Correspondingly, green light is absorbed by a magenta ink, while blue light is absorbed by a yellow ink. Further, each of the filters will tend to absorb light energy at frequencies outside of the bandwidth representative of the particular color hue of the filter. For example, the red filter 18 for cyan indication will tend to absorb all light rays, except for those within the spectral bandwidth corresponding to a red hue. By detecting reflected light rays only within a particular color hue bandwidth, and obtaining an optical density measurement with respect to the same, a "figure of merit" can be obtained with respect to the quality of the object sample coloring associated with that particular color hue.
It is apparent from the foregoing that the actual quantitative measurement of color density or color reflectance is dependent in substantial part on the spectral transmittance characteristics of the filters. Accordingly, various well-known standards have been developed with respect to spectral characteristics of densitometer filters. For example, one standard for densitometer filters is known as the ANSI status T color response. The spectral response characteristics of filters meeting this standard are relatively wide band (in the range of 50-60 namometers (nms) bandwidth) for each of the cyan, magenta and yellow color hues. Other spectral response characteristic standards include, for example, what is known as G-response, which is similar to status T, but is somewhat more sensitive to respect to yellow hues. An E-response represents a European response standard.
Although the filters 18, 20 and 22 are illustrated in the embodiment shown in FIG. 1 as the cyan, magenta and yellow color shades, other color shades can clearly be employed. These particular shades are considered somewhat preferable in view of their relative permanence, and because they comprise the preferred shades for use in reflection densitometer calibration. However, it is apparent that different shades of red, green and blue, or cyan, magenta and yellow, as well as entirely different colors, can be utilized with the densitometer apparatus 1.
The spectral filters 18, 20 and 22 may not only comprise various shades of color, but can also be one of a number of several specific types of spectral response filters. For example, the filters can comprise a series of conventional Wratten gelatin filters and infrared glass. However, various other types of filter arrangements can also be employed.
The spectral filters 18, 20 and 22 are preferably positioned at a 45.degree. angle relative to the normal direction from the plane of the object sample 12 under test. In the particular example shown in FIG. 1, each of these filters is utilized to simultaneously receive light rays reflected from the object sample 12. Further, although the particular example illustrated in FIG. 1 may include a stationary object sample 12 and stationary apparatus 1, the example embodiment of a densitometer apparatus employing principles of the invention as described in subsequent paragraphs herein can comprise a series of stationary object samples (in the form of a color control bar) with movement of the densitometer apparatus so as to "scan" the object samples. In this type of arrangement, the spectral filter arrangement is continuously moving during color measurements of the object samples. In other known densitometers, the spectral filter measurements may be obtained in sequence, rather than simultaneously, and with or without relative movement of the object samples and densitometer apparatus.
As further shown in FIG. 1, the portion of the reflected light rays 14 passing through the filters 18, 20 and 22 (shown as light rays 24, 26 and 28, respectively) impinge on receptor surfaces of photovoltaic sensor cells. The sensor cells are illustrated in FIG. 1 as sensors 32, 34 and 36 associated with the spectral filters 18, 20 and 22, respectively. The sensors 32, 34 and 36 can comprise conventional photoelectric elements adapted to detect light rays emanating through the corresponding spectral filters. The sensors are further adapted to generate electrical currents having magnitudes proportional to the intensities of the sensed light rays. As illustrated in FIG. 1, electrical current generated by the cyan sensor 32 in response to the detection of light rays projecting through the filter 18 is generated on line pair 38. Correspondingly, electrical current generated by the magenta sensor 34 is applied to the line pair 40, while the electrical current generated by the yellow sensor 36 is applied as output current on line pair 42. Photoelectric elements suitable for use as sensors 36, 38 and 40 are wellknown in the art, and various types of commercially available sensors can be employed.
The magnitude of the electrical current on each of the respective line pairs will be proportional to the intensity of the reflected light rays which are transmitted through the corresponding spectral filter. These light rays will have a spectral distribution corresponding in part to the product of the spectral reflectance curve of the object sample 12, and the spectral response curve of the corresponding filter. Accordingly, for a particular color shade represented by the spectral response curve of the filter, the magnitude of the electrical current represents a quantitative measurement of the proportional reflectance of the object sample 12 within the frequency spectrum of the color shade.
As further shown in FIG. 1, the sensor current output on each of the line pairs 38, 40 and 42 can be applied as an input signal to one of three conventional amplifiers 44, 46 and 48. The amplifier 44 is responsive to the current output of cyan sensor 32 on line pair 38, while amplifier 46 is responsive to the sensor current output from magenta sensor 34 on line pair 40. Correspondingly, the amplifier 48 is responsive to the sensor current output from yellow sensor 36 on line pair 42. Each of the amplifiers 44, 46 and 48 provides a means for converting low level output current from the respective sensors on the corresponding line pairs to voltage level signals on conductors 50, 52 and 54, respectively. The voltage levels of the signals on their respective conductors are of a magnitude suitable for subsequent analog-to-digital (A/D) conversion functions. Such amplifiers are well known in the circuit design art, and are commercially available with an appropriate volts per ampere conversion ratio, bandwidth and output voltage range. The magnitudes of the output voltages on lines 50, 52 and 54 again represent the intensities of reflected light rays transmitted through the corresponding spectral filters.
Each of the voltage signal outputs from the amplifiers can be applied as an input signal to a conventional multiplexer 56. The multiplexer 56 operates so as to time multiplex the output signals from each of the amplifiers 44, 46 and 48 onto the conductive path 58. Timing for operation of the multiplexer 56 can be provided by means of clock signals from master clock 60 on conductive path 62. During an actual density measurement of an object sample, the densitometer 1 will utilize a segment of the resultant multiplexed signal which sequentially represents a voltage output signal from each of the amplifiers 44, 46 and 48.
The resultant multiplexed signal generated on the conductive path 58 is applied as an input signal to a conventional A/D converter 64. The A/D converter 64 comprises a means for converting the analog multiplexed signal on conductor 58 to a digital signal for purposes of subsequent processing by central processing unit (CPU) 66. The A/D converter 64 is preferably controlled by means of clock pulses applied on conductor 68 from the master clock 60. The clock pulses operate as "start" pulses for performance of the A/D conversion. The A/D converter 64 can be any suitable analog-to-digital circuit well known in the art and can, for example, comprise 16 binary information bits, thereby providing a resolution of 65 K levels per input signal.
The digital output signal from the A/D converter 64 can be applied as a parallel set of binary information bits on conductive paths 70 to the CPU 66. The CPU 66 can provide several functions associated with operation of the densitometer apparatus 1. The CPU 66 can be utilized to perform these functions by means of digital processing and computer programs. In addition, the CPU 66 can be under control of clock pulses generated from the master clock 60 on path 72. However, a number of the functional operations of CPU 66 could also be provided by means of discrete hardware components.
In part, the CPU 66 can be utilized to process information contained in the digital signals from the conductive paths 70. Certain of this processed information can be generated as output signals on conductive path 76 and applied as input signals to a conventional display circuit 78. The display circuit 78 provides a means for visual display of information to the user, and can be in form of any one of several well-known and commercially-available display units. However, in an embodiment of a scanning densitometer in accordance with the invention as described in subsequent paragraphs herein, a display unit may not be directly associated with the densitometer apparatus, but instead color measurement data may be transmitted from a densitometer-based processor to another computer system, where the other computer system includes means for analyzing and/or displaying or printing data associated with the color measurements.
In addition to the CPU 66 receiving digital information signals from the conductive paths 70, information signals can also be manually input and applied to the CPU 66 by means of a manually-accessible keyboard circuit 80. The user can supply "adjustments" to color responses and various data parameters by means of entering information through the keyboard 80. Signals representative of the manual input from the keyboard 80 can be applied as digital information signals to the CPU 66 by means of conductive path 82. Again, however, in an embodiment of a scanning densitometer in accordance with the invention as described in subsequent paragraphs herein, a keyboard or similar data entry device may not be directly associated with the densitometer-based processor. Instead, data input to the densitometer apparatus may be provided by data entry devices associated with separate and/or remote computer systems having a communications interface with the densitometer apparatus 1. Concepts associated with providing a communications interface between a densitometer-based computer and an external or remote computer system are disclosed in Peterson et al, U.S. Pat. No. 4,591,978 issued May 27, 1986.
The previously described concepts of densitometry and densitometer apparatus in general can be of primary significance in various industries, including the printing and graphic arts industries. For example, densitometers conventionally known as "scanning" densitometers are typically utilized for analysis of color control bars printed on press sheets so as to analyze the color printing and reproduction, and ensure maintenance of color quality. More specifically, known scanning densitometers can sequentially measure color bar "patches" comprising color data representative of solids, screened areas, overprints, etc. Through analysis of these color patches, the densitometers can typically be utilized to provide specific density data, in addition to analyzed data such as density differences and the like. Other parameters or quantities which may be obtained through use of scanning densitometers include dot percentage/gain, relative print contrast, trappings, grayness, hue error and various statistical production data. Known and commercially available scanning densitometer arrangements include apparatus known as the Autosmart.TM. Densitometer marketed by Cosar Corporation, the Gretag D732 Densitometer marketed by Gretag Limited and the Tobias SCR Densitometer marketed by Tobias Associates, Inc.
The known scanning densitometers typically include relatively complex and large scanning "heads" comprising the electronics and similar apparatus required for optically obtaining color density data. In addition, with a scanning densitometer, the head is typically mounted in a manner so that it is movable along a carriage or the like so as to sequentially obtain color density measurement data from a series of color bar patches positioned on a stationary print sheet. With many of the known scanning densitometers, the scanning heads are continuously in electrical communication with computer processor and memory configurations so as to transmit parameter data and color measurement data between the scanning head electronics and a separate and/or remote computer system. To provide for this electrical communication, many of the known scanning densitometers include cabling interconnections between the scanning head and separate computer-based apparatus.
The electrical cabling required for communication interconnections between the scanning head and separate computer-based apparatus can be of substantial weight. Accordingly, to provide for movement of the scanning head, several known densitometers require relatively complex track, gearing and motor control arrangements for providing the scanning head movement in response to externally initiated commands. The known systems can require a substantial amount of power in view of their motor-driven and cabling characteristics. Also, for purposes of obtaining accurate measurements, many of the known densitometers utilize vacuum systems or other relatively elaborate "hold down" arrangements for purposes of clamping the color bar paper along a flat surface during the measurement cycles. In view of all of the foregoing requirements, many of the known densitometers are also relatively expensive.